Food packaging materials are required to have both good sealing properties and good openability permitting easy seal failure when the contents are taken out.
Polypropylene films or the like are conventionally used in this field. Patent Document 1 discloses a co-extruded multilayer film wherein a laminating resin layer (A) containing a propylene resin (a) is laminated adjacent to a heat seal resin layer (B) having a thickness of 1 to 15 μm which contains a propylene resin (b1) and a high density polyethylene (b2) with a density of not less than 0.955 g/cm3 in a total amount of not less than 95 wt % and in a mass ratio (b1)/(b2) of 40/60 to 75/25.
The patent document describes that the film can be heat sealed easily and stably over a wider range of temperatures, can withstand breakage at the heat sealed part in retort sterilization or the like, and can be easily opened without delamination.
When oil-rich contents are packaged in these easy-open films and are retort sterilized, however, oil penetrates into the films to cause unstable peeling such as separation (delamination) or stringing of a cohesive failure heat seal layer.
Patent Document 2 discloses a multilayer laminate film wherein a first layer is a resin layer comprising a resin composition which is obtained from a high-pressure low-density polyethylene resin having MFR of 1 to 7 g/10 min and a polypropylene resin having MFR of 15 to 30 g/10 min as main components in a ratio of 20 to 60 parts by weight for the former resin and 40 to 80 parts by weight for the latter resin, and a second layer is composed of one, or two or more resin layers comprising a resin composition which is obtained from a single site catalyzed ethylene/α-olefin copolymer, a high density polyethylene having a density of not less than 0.935 g/cm3 or a polypropylene resin as a main component.
In Example 5 of Patent Document 2, a co-extruded multilayer laminate film having different three layers is described wherein the first layer is a resin layer comprising a resin composition from 40 parts by weight of a high-pressure low-density polyethylene resin and 60 parts by weight of a polypropylene copolymer, the second layer is a resin layer comprising a single site catalyzed ethylene/α-olefin copolymer, and the third layer is a resin layer comprising a polypropylene copolymer.
However, stable heat sealing properties cannot be obtained when oil-rich contents are packaged in laminate films that include a first layer comprising a resin composition from 40 parts by weight of a high-pressure low-density polyethylene resin, and a second and a third layer comprising an ethylene/α-olefin copolymer and a polypropylene copolymer respectively. In retort applications in particular, even when such films are designed to achieve the desired heat seal strength, the actual heat seal strength tends to be greatly variable and stable performances cannot be obtained. This tendency is probably because oil contained in oil-rich foods penetrates into the laminate films during retort sterilization and increases the probability of separation (delamination) or stringing of a cohesive failure heat seal layer.
Meanwhile, the melting point of linear low density polyethylenes (L-LDPE) is proportional to the density of the resins, and the higher the density, the higher the melting point. When they are retort processed at high temperatures, however, they may be partially molten and be delaminated from a substrate such as polyester (PET) or nylon (Ny), or the films may be clouded by recrystallization similar to high density polyethylenes (HDPE).
Low density polyethylenes (LDPE) generally have a melting point of about 110° C. When they are subjected to retort processing and sterilization near or above their melting points, they will be molten and be delaminated from a substrate such as polyester (PET) or nylon (Ny).
The melting point of high density polyethylenes (HDPE) is generally about 132° C. When they are retort processed at high temperatures of 127° C. or above, they may be partially molten and be delaminated from a substrate such as polyester (PET) or nylon (Ny), or the films may be clouded by recrystallization.
Patent Document 3 discloses a lid material using a co-extruded multilayer laminate film that is obtained similar to Patent Document 2 except that the polypropylene resin having MFR of 15 to 30 g/10 min for the first layer is replaced by a propylene/ethylene random copolymer having MFR of 1 to 15 g/10 min.
In Example 5 of Patent Document 3, a co-extruded multilayer laminate film having different three layers is described wherein the first layer is a resin layer comprising a resin composition from 30 parts by weight of a high-pressure low-density polyethylene resin and 70 parts by weight of a propylene/ethylene random copolymer, the second layer is a resin layer comprising a polypropylene copolymer, and the third layer is a resin layer comprising a single site catalyzed ethylene/α-olefin copolymer.
However, stable heat sealing properties cannot be obtained when oil-rich contents are packaged in laminate films that include a first layer comprising a resin composition from 30 parts by weight of a high-pressure low-density polyethylene resin, and a second and a third layer comprising a polypropylene copolymer and an ethylene/α-olefin copolymer. In retort applications in particular, even when such films are designed to achieve the desired heat seal strength, the actual heat seal strength tends to be greatly variable and stable performances cannot be obtained.
This tendency is probably because oil contained in oil-rich foods penetrates into the laminate films during retort sterilization and increases the probability of separation (delamination) or stringing of a cohesive failure heat seal layer.